A compound contains only nitrogen and hydrogen and is 87.4% nitrogen by mass. A gaseous sample of the compound has a density of 0.977 g/L at 680. torr and 84.1°C. What is the molecular formula of the compound?

Take a 100 g sample. That will give you
87.4 g nitrogen
100-87.4 g hydrogen.

Convert grams to moles. moles = g/atomic mass.
87.4/14 = ??
12.6/1 = ??
Find the ratio of the two atoms with the smallest number being 1. The easy way to do that is to divide the smaller number by itself; therefore,that number will be 1.000. Divide the other number by the same small number. That gives you the formula of
NxHy. The empirical mass can be calculated from that.

The second part of the problem is to determine the molar mass of the gas. That can be done by using a modified gas law of P*molar mass = density*R*T
Solve for molar mass.
Divide molar mass/empirical mass, round to a whole number and that will be z in the following.
(NxHy)z

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